This Blog is intended to collect information of my various Intrests,pen my opinion on the information gathered and not intended to educate any one of the information posted,but are most welcome to share there view on them

Monday, February 13, 2012

Securing and Hardening Red Hat Linux Production Systems

Physical security should be of the utmost concern. Linux production servers should be in locked datacenters where only people with passed security checks have access. But physical security is out of scopy for this article. Depending on the environment and circumstances, you may want to consider boot loader passwords.

It is strongly recommended to have scripts available which verify that all security action items have been executed. Even the best sysadmins can make mistakes and miss steps. If you have a larger Linux environment, it would be a good investment to write scripts for checking Linux security action items.

To retire servers with sensitive data, it is important to ensure that data cannot be recovered from the hard disks. To ensure that all traces of data are removed, the DiskSanitizer tool can be used. This tool can be operated from a floppy disk and it removes data according with the U.S. Department of Defense (DoD) standards. DiskSanitizer is available at http://freshmeat.net/projects/disksanitizer.

If your system gets compromised, your backups become invaluable. But also in cases like bugs, accidents etc. backups can be used to compare you current system against your backed-up system. For production systems it is very important to take some Backups offsite for cases like disasters.

For legal reasons, some firms and organizations must be careful about backing up too much information and holding it too long. If your environment has a policy regarding the destruction of old paper files, you might have to extend this policy to Linux backup tapes as well.

Servers should have separate partitions for at least /, /boot, /usr, /var, /tmp, and /home. You don't want that e.g. logging and temporary space under /var and /tmp fill up the root partition. Third party applications should be on separate filesystems as well, e.g. under /opt.

I will not cover iptables in this paper. Most companies use hardware based firewalls to protect their servers in a production network, which is strongly recommended for such environments. If you are interested in a Linux Stateful Firewall using iptables, check out my HOWTO for Stateful Firewall and Masquerading on Linux. For lots of iptables tutorials and examples, see http://www.linuxguruz.com/iptables/.

SELinux is an advanced technology for securing Linux systems. Hardening Linux using SELinux technology, on its own, warrants it's own security HOWTO and is out of scope for this guide. I highly recommend the book SELinux: NSA's Open Source Security Enhanced Linux.

FTP, telnet, and rlogin (rsh) are vulnerable to eavesdropping, which is one of the reasons why SSH/SCP/SFTP should be used instead. It is highly recommended not to run these services. Due to the high risk, this guide does not cover these services. It would also be a good idea not to have FTP and Telnet server RPMs installed on the system.

Removing Unnecessary Software Packages (RPMs)

A very important step in securing a Linux system is to determine the primary function or role of the Linux server. You should have a detailed knowledge of what is on your system. Otherwise you will have a difficult time to understand what needs to be secured and hence securing your Linux systems proactively won't be that effective. Therefore, it is very critical to look at the default list of software packages and remove unneeded packages or packages that don't comply with your security policy. If you do that you will have less packages to update and to maintain when security alerts and patches are released. For example, you should not have Apache or Samba installed on your system if you don't use them. Also, it is a good practice not to have development packages, desktop software packages (e.g. X Server) etc. installed on production servers. Other packages like FTP and Telnet daemons should not be installed as well unless there is a justified business reason for it (SSH/SCP/SFTP should be used instead).

One of the first action items should be to create a Linux image that only contains RPMs needed by the applications, and needed for maintenance and troubleshooting purposes. A good approach is to start with a minimum list of RPMs and then add packages as needed. It may be time-consuming but worth the efforts.

To get a list of all installed RPMs you can use the following command:

rpm -qa

If you want to know more about a particular RPM, run:

rpm -qi

To check for and report potential conflicts and dependencies for deleting a RPM, run:

rpm -e --test

For information on performing Kickstart installations and how to build an image, see Kickstart Installations for more information.

Patching Linux Systems

Building an infrastructure for patch management is another very important step to proactively secure Linux production environments. It is recommended to have a written security policy and procedure to handle Linux security updates and issues.

For example, a security policy should detail the timeframe for assessment, testing, and rollout of patches. Network related security vulnerabilities should get the highest priority and should be addressed immediately within a short timeframe.

For example, a security procedure should detail the process for assesment, testing, and rollout of patches. The assessment phase should occur within a testing lab, and initial rollout should occur on development systems first.

A separate security log should detail what Linux security notices have been received, when patches have been researched and assessed, when patches have been applied etc.

From the output you can see that xinetd, sendmail, and sshd are listening.

On all newer Red Hat Linux distributions sendmail is configured to listen for local connections only. Sendmail should not listen for incoming network connections unless the server is a mail or relay server. Running a port scan from another server will confirm that (make sure that you have permissions to probe a machine):

Note that the above nmap command can take a while. If you remove the UDP port scan (without the option "-U"), then nmap will finish the port scan immediately. If you run it on the local machine it will also complete very fast. Also note that nmap might not show all listening network sockets if a firewall is being used to block ports. From the output above you can see that the xinetd daemon is listening on port auth (port 113) for IDENT (for more information on this service, see below). You can also see that sendmail is not listening for remote incoming network connections, see also Securing Sendmail.

Another method to list all of the TCP and UDP sockets to which programs are listening is lsof:

Closing Network Ports and Disabling Runlevel System Services

One of the most important tasks is to remove any network services from the system startup process that are not needed.

On Red Hat systems you can list all services which are started at bootup using the following command:

chkconfig --list |grep on

You will notice that there are quite a lot of services enabled on your system. But many runlevel services (Stand-Alone Services) are not network related services like kudzu which is responsible for detecting and configuring new and/or changed hardware on your system. This service is only run during the boot process. Ensure not to disable runlevel services that are needed by the system to run smoothly.

Here are examples of Red Hat Runlevel System Services which you may or may not want to enable:

gpm needed if you want to use the mouse at the console
kudzu important for detecting new hardware
syslog important for syslog services
netfs needed only if there are NFS shares that should be mounted at boot time
network important for starting network interfaces (e.g. eth0, eth1, bonding,...)
random used for the system entropy pool
atd needed if the at(1) service is used instead of cron
apmd Advanced Power Management (APM) daemon is used for laptops and some desktops
isdn needed if ISDN is being used
iptables needed if Netfilter (iptables) Firewall is being used
ip6tables needed if ip6tables Firewall is being used
pcmcia not needed on servers - needed for laptops
irqbalance important for distributing interrupts across all CPUs
sendmail needed if Sendmail is used - Procmail should be used which is more secure
autofs needed if automounter is used - production applications should not be dependent on automounter
sshd important for logins via SSH
portmap needed if e.g. NFS is being used
nfslock needed if NFS shares are mounted
nfs needed if server runs the NFS server
mdmonitor needed only if software RAID is being used
crond important for running cron jobs
xinetd needed if xinetd services are being used, see /etc/xinetd.d/ for list of services
cups needed if CUPS is used for the printing system
rhnsd needed if server should connect to RHN to check for software updates etc.
sysstat needed to reset system statistics logs
audit needed only if Linux Audit Subsystem (LAuS) should run for collecting system call audit records
psacct needed only if kernel process accounting information is needed
smartd important for monitoring disk problems if hard disks support SMART technology
netdump important if kernel oops data and memory dumps should be sent to a Netdump server for server crashes

The start/stop scripts of all runlevel services can be found in the /etc/init.d directory. For example, if you don't know what the atd service does, go to /etc/init.d and open the file atd. And in the script look for lines that start programs. In the atd script the "daemon /usr/sbin/atd" line starts the binary atd. Now having the name of the program that is started by this service, you can check the online pages of atd by running man atd. This will help you to find out more about a system service.

To permanently disable e.g. the runlevel service nfs, run:

chkconfig nfs off

To immediately disable the runlevel service nfs, run:

/etc/init.d/nfs stop

Closing Network Ports and Disabling Xinetd Services

The xinetd daemon is a replacement for inetd, the internet services daemon. It monitors the ports for all network services configured in /etc/xinetd.d, and starts the services in response to incoming connections.

If xinetd is active, it is important to check which Unix services are active and controlled by xinetd. The following command shows all services configured in /etc/xinetd.d and wheter xinetd monitors the ports for these services:

# chkconfig --list | awk '/xinetd based services/,/""/'
xinetd based services:
krb5-telnet: off
rsync: off
eklogin: off
gssftp: off
klogin: off
chargen-udp: off
kshell: off
auth: on
chargen: off
daytime-udp: off
daytime: off
echo-udp: off
echo: off
services: off
time: off
time-udp: off
cups-lpd: off
#

To get a list of only active services for which xinetd monitors the ports, you could run:

In the above example you can see that the telnet-server RPM is not installed on the system. If the Telnet Server package telnet-server would be installed, it would show up on the list whether it's active or not.

Here is an example how to disable a service. Assuming the telnet service is active, run the following commands to disable it and to see how the telnet service entries are being updated:

This example shows what can be done if there exists no online manuals for the binary in.authd that is started by xinetd. The steps above should be helpful for finding out more about services.

The auth service (aka IDENT, see RFC 1413) allows remote daemons to query information about users establishing TCP connections on the local server. In a trusted environment it helps a server to identify who is trying to use it. For example, it can provide vital information for troubleshooting and who has done what. IDENT requests are needed by some applications like IRC. However, IDENT can be a security risk.

To disable the auth service, run the following command:

# chkconfig auth off

The xinetd daemon is quite flexible and has many features. Here are just a few functionalities of Xinetd:- Acces control for TCP, UDP, and RPC services- Acess limitations based on time- Provides mechanisms to prevent DoS attacks

Reviewing Inittab and Boot Scripts

The inittab file /etc/inittab also describes which processes are started at bootup and during normal operation. For example, Oracle uses it to start cluster services at bootup. Therefore, it is recommended to ensure that all entries in/etc/inittab are legitimate in your environment.

I would at least remove the CTRL-ALT-DELETE trap entry to prevent accidental reboots:

# sed -i 's/ca::ctrlaltdel:/#ca::ctrlaltdel:/g'

The default runlevel should be set to 3 since in my opinion X11 (X Windows System) should not be running on a production server. In fact, it shouldn't even be installed.

# grep ':initdefault' /etc/inittab
id:3:initdefault:
#

And depending on your environment you might want to comment out the UPS entries as well.

To have changes in /etc/inittab become effective immediately, you can run:

# init q

The /etc/rc.local script is used for commands or startup scripts which are pertinent only to a specific server. (/etc/rc.local is a link to /etc/rc.d/rc.local). Ensure that all startup scripts in /etc/rc.d/rc.local are legitimate.

Restricting System Access from Servers and Networks

Usually a firewall is used to protect a server from other servers and networks. However, in some cases you may also want to protect a server within a network by using a TCP Wrapper.

The Xinetd super server that comes with most Linux distributions includes a built-in TCP wrapper. It can be used to explicitly define network services to accept incoming connections from specified servers and networks. The TCP wrappers implements access control through the use of two files, /etc/hosts.allow and /etc/hosts.deny. Note that the hosts.allow file takes precedence over the hosts.deny file. And you may want to change the permissions on the two configuration files since they are both world readable.

A recommended security-strategy is to block all incoming requests by default, but allow specific hosts or networks to connect. This is the strategy I will describe here.

To deny everything by default, add the following line to /etc/hosts.deny:

ALL: ALL

To accept incoming SSH connections from e.g. nodes rac1cluster, rac2cluster and rac3cluster, add the following line to /etc/hosts.allow:

sshd: rac1cluster rac2cluster rac3cluster

To accept incoming SSH connections from all servers from a specific network, add the name of the subnet to /etc/hosts.allow. For example:

sshd: rac1cluster rac2cluster rac3cluster .subnet.example.com

To accept incoming portmap connections from IP address 192.168.0.1 and subnet 192.168.5, add the following line to /etc/hosts.allow:

portmap: 192.168.0.1 192.168.5.

To accept connections from all servers on subnet .subnet.example.com but not from server cracker.subnet.example.com, you could add the following line to /etc/hosts.allow:

ALL: .subnet.example.com EXCEPT cracker.subnet.example.com

Here are other examples that show some features of TCP wrapper:

If you just want to restrict ssh connections without configuring or using /etc/hosts.deny, you can add the following entries to /etc/hosts.allow:

sshd: rac1cluster rac2cluster rac3cluster
sshd: ALL: DENY

The version of TCP wrapper that comes with Red Hat also supports the extended options documented in the hosts_options(5) man page. Here is an example how an additional program can be spawned in e.g. the /etc/hosts.allowfile:

The TCP wrapper is quite flexible. And xinetd provides its own set of host-based and time-based access control functions. You can even tell xinetd to limit the rate of incoming connections. I recommend reading various documentations about the Xinetd super daemon on the Internet.

Many network services like telnet, rlogin, and rsh are vulnerable to eavesdropping which is one of several reasons why SSH should be used instead. Red Hat's default configuration for SSH meets the security requirements for many environments. However, there are a few parameters in /etc/ssh/sshd_config that you may want to change on RHEL and other Linux systems.

The chapter Restricting System Access from Servers and Networks shows how direct logins can be disabled for shared and system accounts including root. But it's prudent to disable direct root logins at the SSH level as well.

PermitRootLogin no

Also ensure to have privilege separation enabled where the daemon is split into two parts. With privilege separation a small part of the code runs as root and the rest of the code runs in a chroot jail environment. Note that on older RHEL systems this feature can break some functionality, for example see Preventing Accidental Denial of Service.

UsePrivilegeSeparation yes

Since SSH protocol version 1 is not as secure you may want to limit the protocol to version 2 only:

Protocol 2

You may also want to prevent SSH from setting up TCP port and X11 forwarding if you don't need it:

AllowTcpForwarding no
X11Forwarding no

Ensure the StrictModes directive is enabled which checks file permissions and ownerships of some important files in the user's home directory like ~/.ssh, ~/.ssh/authorized_keys etc. If any checks fail, the user won't be able to login.

StrictModes yes

Ensure that all host-based authentications are disabled. These methods should be avoided as primary authentication.

IgnoreRhosts yes
HostbasedAuthentication no
RhostsRSAAuthentication no

Disable sftp if it's not needed:

#Subsystem sftp /usr/lib/misc/sftp-server

After changing any directives make sure to restart the sshd daemon:

/etc/init.d/sshd restart

Securing Postfix

Postfix is a replacement for Sendmail which has several security advantages over Sendmail. Postfix consists of several small programs that perform their own small task. And almost all programs run in a chroot jail. These are just a few examples why Postfix is recommended over Sendmail. For more information on chroot jail, see Using Chroot Securely.

Linux servers that are not dedicated mail or relay servers should not accept external emails. However, it is important for production servers to send local emails to a relay server.

Before you continue on a Red Hat system, make sure Postfix is activated using the following command:

# alternatives --set mta /usr/sbin/sendmail.postfix

The following parameters in /etc/postfix/main.cf should be set to ensure that Postfix accepts only local emails for delivery:

Once you've configured Postfix, restart the mail system with the following command:

# /etc/init.d/postfix restart

To verify whether Postfix is still listening for incoming network request, you can run one of the following commands from another node:

# nmap -sT -p 25
# telnet 25

Don't run these commands on the local host since Postfix is supposed to accept connections from the local node.

If you believe that I should cover other parameter(s) or if you think that other parameter(s) should explicitly be set/changed for local mail delivery, please drop me an email.

Securing Sendmail

This article focuses on security issues that pertain to most Linux servers in a production environment. Therefore, securing a mail or relay server is out of scope for this article since not all Linux servers in a production environment are mail or relay servers. However, Sendmail or Postfix is usually required for local mail delivery. Note that it is recommended to use Postfix over Sendmail for various security reasons, see Securing Postfix for more information.

On newer Linux systems Sendmail is configured to run in the background for local mail delivery and not to accept incoming network connections. If your server is not a mail or relay server, then it is important that Sendmail is not accepting incoming network connections from any host other than the local server.

The default sendmail.cf configuration file on RedHat does not allow Sendmail to accept incoming network connections. The following setting in /etc/mail/sendmail.cf tells Sendmail not to accept incoming network connections from servers other than the local node:

O DaemonPortOptions=Port=smtp,Addr=127.0.0.1, Name=MTA

If that's not the case on your system, you can change it by setting or uncommenting the DAEMON_OPTIONS parameter in the /etc/mail/sendmail.mc file. Uncomment the DAEMON_OPTIONS line in /etc/mail/sendmail.mc to read:

Securing NFS

NFS (Network File System) allows servers to share files over a network. But like all network services using NFS involves risks. Here are some basic rules:- NFS should not be enabled if not needed.- If you must use NFS, use TCP wrapper to restrict remote access.- Make sure you export to only those machines that you really need to.- Use fully qualified domain names to diminish spoofing attempts.- Export only directories you need to export.- Export read-only wherever possible.- Use NFS over TCP.

If you don't have shared directories to export, ensure that the NFS service is NOT enabled and running:

If you must use NFS, it can be activated using the following commands on Red Hat Linux:

chkconfig portmap on
chkconfig nfs on
service portmap start
service nfs start

The "portmap" service starts the portmap daemon. Depending on the Linux OS distribution and version, the "nfs" service starts the rpc.rquotad, nfsd, lockd, rpciod, rpc.mountd and rpc.idmapd daemons on Red Hat Fedora Core 3 which is the OS that Red Hat Advanced Server 4 is based on. On Red Hat Advanced Server 3, the NFS service starts rpc.mountd, nfsd and rpc.rquotad.

To probe the portmapper for all registered NFS related RPC programs, you can run rpcinfo. On Red Hat Advanced Server 3, the output will look like this:

As I showed at Restricting System Access from Servers and Networks a recommended security-strategy is to block all incoming requests by default, but allow specific hosts or networks to connect. This is the strategy I will use here.

The portmap program and some of the NFS programs include a built-in TCP wrapper. To verify if a program includes a TCP wrapper, you can run the following commands:

If hosts.deny and hosts.allow are displayed, or if libwrap is displayed, then the program includes a built-in TCP wrapper. If none of these strings are displayed, then adding the program name to /etc/hosts.deny and/etc/hosts.allow will most probably have no effect.

To block all incoming requests by default, add the following line to /etc/hosts.deny if you have not done so yet:

ALL: ALL

Verify from a remote server that portmapper does not list any registered RPC programs:

# rpcinfo -p
No remote programs registered.
#

To allow NFS mounts from trusted servers and networks only, you have to configure the servers and networks for portmap and various NFS related programs in /etc/hosts.allow. To find out which NFS programs are invoked when you start the NFS server, you can check the /etc/init.d/nfs service script. Then run the "strings | egrep "hosts.deny|hosts.allow|libwrap"" command I described above.

On Red Hat Advanced Server 3, the /usr/sbin/rpc.rquotad program includes a built-in TCP wrapper. On Red Hat Fedora Core 3, /usr/sbin/rpc.mountd now also includes a built-in TCP wrapper.

To allow NFS requests from e.g. servers rac1pub.example.com, rac2pub.example.com, rac3pub.example.com and from the .subnet.puschitz.com network, the configuration in /etc/hosts.allow would look like as follows:

Note that options MUST NOT be separated from hostnames or networks with whitespace(s).And use fully qualified domain names to diminish spoofing attempts.

All entries in /etc/exports are exported with the root_squash option ('root squashing') by default. This means that a root user on a client machine does not have root privileges (root access) to root-owned files on exported NFS filesystems/directories. It is not recommended to turn 'root squashing" off using the no_root_squash option!

After you've made all your entries in /etc/exports, you can export all filesystems/directories using the following command:

If you need NFS, it is recommended to use NFS over TCP since NFS over UDP is not very secure. All 2.4 and 2.6 kernels support NFS over TCP on the client side. Server support for TCP appears in later 2.4 kernels, and in all 2.6 kernels.

To verify whether your server supports NFS over TCP, use the wire-test command (/usr/sbin/wire-test is part of the am-utils package). If your server supports NFS over TCP, the output looks like this:

Copying Files Using SSH Without Providing Login Prompts

The following example is a suggestion you may want to use in some cases. It shows how files can be copied over the network using ssh without providing an interactive login prompt.

SSH allows you to do a forced command using the "command" option. When you use this option you can disable scp and enforce every passed ssh command to be ignored. On the server side where you want to retrieve the file from, add the following entry to the beginning of the SSH key in the ~/.ssh/authorized_keys2 file:

command="/bin/cat ~/" ssh-dss AAABB33Nza...OpenSSH key

To copy now the file from the remote server, you can run the following command:

ssh @ >

Since /bin/cat is executed on the server side, the output has to be redirected to the local file.

Another approach is to replace /bin/cat with your own script that checks the passed SSH commands by reading the environment variable $SSH_ORIGINAL_COMMAND. For example:

So you replace /bin/cat with the script name in ~/.ssh/authorized_keys2, and run the following command to copy "File1":

ssh @ File1 >

To copy "File 2", run:

ssh @ File2 >

Every other passed parameter will return an error.

Kernel Tunable Security Parameters

The following list shows tunable kernel parameters you can use to secure your Linux server against attacks.

For each tunable kernel parameters I will show the entry that needs to be added to the /etc/sysctl.conf configuration file to make the change permanent after reboots. To activate the configured kernel parameters immediately at runtime, use:

Source Routing is used to specify a path or route through the network from source to destination. This feature can be used by network people for diagnosing problems. However, if an intruder was able to send a source routed packet into the network, then he could intercept the replies and your server might not know that it's not communicating with a trusted server.

To enable Source Route Verification, edit the /etc/sysctl.conf file and add the following line:

ICMP redirects are used by routers to tell the server that there is a better path to other networks than the one chosen by the server. However, an intruder could potentially use ICMP redirect packets to alter the hosts's routing table by causing traffic to use a path you didn't intend.

To disable ICMP Redirect Acceptance, edit the /etc/sysctl.conf file and add the following line:

IP spoofing is a technique where an intruder sends out packets which claim to be from another host by manipulating the source address. IP spoofing is very often used for denial of service attacks. For more information on IP Spoofing, I recommend the article IP Spoofing: Understanding the basics.

To enable IP Spoofing Protection, turn on Source Address Verification. Edit the /etc/sysctl.conf file and add the following line:

Checking File Permissions and Ownership

The umask (user file-creation mode mask) command is a shell built-in command which determines the default file permissions for newly created files. This can be overwritten by system calls but many programs and utilities make use of umask.

By default, Red Hat sets umask to 022 or 002 which is fine. If the name of the user account and the group account is the same and the UID is 100 or larger, then umask is set to 002, otherwise it's set to 022, see /etc/bashrc forbash shells.

When the SUID (set user ID) or SGID (set group ID) bits are set on an executable, it executes with the UID or GID of the owner of the executable rather than that of the person executing it. This means that e.g. all executables that have the SUID bit set and are owned by root are executed with the UID of root. A good example is the passwd command that allows ordinary users to update the password field in the /etc/shadow file which is owned by root.

But SUID/SGID bits can be misused when the SUID/SGID executable has a security hole. Therefore, you might want to search the entire system for SUID/SGID executables and document it. For example, ensure that code developers don't set SUID/SGID bits on their programs if it's not an absolute requirement. Very often you can use workarounds like removing just the executable bit for world/others. However, a better approach is to change the design of the software if possible.

To search the entire system for SUID or SGID files, you can run the following command:

find / -path /proc -prune -o -type f -perm +6000 -ls

The -prune option in this example is used to skip the /proc filesystem.

World-writable files are a security risk since it allows anyone to modify them. Additionally, world-writable directories allow anyone to add or delete files.

To locate world-writable files and directories, you can use the following command:

find / -path /proc -prune -o -perm -2 ! -type l -ls

The "! -type l" parameter skips all symbolic links since symbolic links are always world-writable. However, this is not a problem as long as the target of the link is not world-writable, which is checked by the above findcommand.

World-Writable directories with sticky bit such as the /tmp directory do not allow anyone except the owner of a file to delete or modify it in this directory. The sticky bit makes files stick to the user who created it and it prevents other users from deleting and renaming the files. Therefore, depending on the purpose of the directory world-writable directories with sticky are usually not an issue. An example is the /tmp directory:

$ ls -ld /tmp
drwxrwxrwt 18 root root 16384 Dec 23 22:20 /tmp

The "t" mode, which denotes the sticky bit, allows files to be deleted and renamed only if the user is the owner of this file or the owner of the directory.

Files not owned by any user or group might not necessarily be a security problem in itself. However, unowned files could pose a security problem in the future. For example, if a new user is created and the new users happens to get the same UID as the unowned files have, then this new user will automatically become the owner of these files.

To locate files not owned by any user or group, use the following command:

find / -path /proc -prune -o -nouser -o -nogroup

Checking Accounts

It is important that all system and vendor accounts that are not used for logins are locked.

To get a list of unlocked accounts on your system, you can check for accounts that do NOT have an encrypted password string starting with "!" or "*" in the /etc/shadow file. If you lock an account using passwd -l, it will put a '!!' in front of the encrypted password, effectively disabling the password. If you lock an account using usermod -L, it will put a '!' in front of the encrypted password. Many system and shared accounts are usually locked by default by having a '*' or '!!' in the password field which renders the encrypted password into an invalid string.

Hence, to get a list of all unlocked (encryptable) accounts, run:

# egrep -v '.*:\*|:\!' /etc/shadow | awk -F: '{print $1}'

Also make sure all accounts have a 'x' in the password field in /etc/passwd. The following command lists all accounts that do not have a 'x' in the password field:

# grep -v ':x:' /etc/passwd

A 'x' in the password fields means that the password has been shadowed, i.e. the encrypted password has to be looked up in the /etc/shadow file. If the password field in /etc/passwd is empty, then the system will not lookup the shadow file and it will not prompt the user for a password at the login prompt.

All system or vendor accounts that are not being used by users, applications, by the system or by daemons should be removed from the system. You can use the following command to find out if there are any files owned by a specific account:

# find / -path /proc -prune -o -user -ls

The -prune option in this example is used to skip the /proc filesystem.

If you are sure that an account can be deleted, you can remove the account using the following command:

# userdel -r

Without the "-r" option userdel will not delete the user's home directory and mail spool (/var/spool/mail/). Note that many system accounts have no home directory.

Some admins suggest to add the following line to the /etc/inittab file to ensure that a root password is required for Single User Mode logons:

~~:S:wait:/sbin/sulogin

This works but can be circumvented easily! At the GRUB or LILO prompt you can tell the boot loader to alternate the init program by using the boot params "init=/bin/bash". This will place you at a root shell prompt without a password.

Enabling Password Aging

In general I do not recommend that the system enforces password expiration for system and shared accounts. This could lead to outages if an application's account expires:

Rather a corporate policy should govern password changes for system and shared accounts. But for individual user accounts the system should expire the passwords automatically. The following example shows how password expiration can be setup for individual user accounts.

The following files and parameters in the table are used when a new account is created with the useradd command. These settings are recorded for each user account in the /etc/shadow file. Therefore, make sure to configure the following parameters before you create any user accounts using the useradd command:

/etc/login.defs

PASS_MAX_DAYS

60

Maximum number of days a password is valid.

/etc/login.defs

PASS_MIN_DAYS

7

Minimum number of days before a user can change the password since the last change.

/etc/login.defs

PASS_MIN_LEN

n/a

This parameter does not work. It is superseded by the PAM module "pam_cracklib". See Enforcing Stronger Passwords for more information.

/etc/login.defs

PASS_WARN_AGE

7

Number of days when the password change reminder starts.

/etc/default/useradd

INACTIVE

14

Number of days after password expiration that account is disabled.

/etc/default/useradd

EXPIRE

Account expiration date in the format YYYY-MM-DD.

Ensure that the above parameters are changed in the /etc/login.defs and /etc/default/useradd files.

When a user account is created using the useradd command, the parameters listed in the above table are recorded in the /etc/shadow file in the following fields:

:::PASS_MIN_DAYS:PASS_MAX_DAYS:PASS_WARN_AGE:INACTIVE:EXPIRE:

To create a new user account you can execute the following command:

useradd -c "Test User" -g users test

The -g option specifies the primary group for this account:

# id test
uid=509(test) gid=100(users) groups=100(users)

The settings in /etc/login.defs and /etc/default/useradd are recorded for the test user in the /etc/shadow file as follows:

# grep test /etc/shadow
test:!!:12742:7:60:7:14::

You can change the password aging any time using the chage command.

To disable password aging for system and shared accounts, you can run the following chage command:

On an audited system it is important to restrict people from using simple passwords that can be cracked too easily. However, if the passwords being enforced are too strong, people will write them down. Strong passwords that are written down are not much safer than weak passwords. Some will argue that strong passwords protect you against e.g. Dictionary Attacks and you can defeat it by locking the accounts after a few failed attempts. However, this is not always an option. As I will show at Locking User Accounts After Too Many Login Failures, locked system accounts could bring down your applications and systems which would be nothing short of a denial of service attack.

Undoubtedly, it is important to practise safe password management. In my opinion, a password should have at least one digit number, one other character, and one upper case letter. But keep in mind not to make it overly complicated.

The pam_cracklib module checks the password against dictionary words and other constraints. Unfortunately, however, the original Linux PAM module pam_cracklib uses a credit mechanism. E.g. if you define password lengthminlen=10, then you will get 1 credit for e.g. using a single digit number in your password if you defined dredit=1. This means that pam_cracklib will accept a password of the length of minlen-credit. If you don't use a digit number, then the minimum length of the password would be minlen. There was no way to tell the module that a password _must_ include a digit number.

Back in 2000 I wrote a patch for the pam_cracklib module where you can assign negative values to the pam_cracklib parameters lcredit, ucredit, dcredit, and ocredit. Using negative values will disable the credit mechanism. For example, if you define dredit=-1, then the module will only accept a password if it includes at least one digit number and if the password has a length of at least minlen.

Red Hat has finally applied my pam_cracklib patch and you don't have to patch the pam_cracklib module any more. The new pam_cracklib feature works in Red Hat Enterprise Linux 4 and Red Hat Fedora Core 3. This feature is now also included with the Red Hat Enterprise Linux 3 Update 4 and Red Hat Enterprise Linux 2.1 Update 6 release. If the Linux distribution you are using does not use the patched pam_cracklib module yet, you can find the procedure for patching pam_cracklibhere.

In the following example I'll assume that you are using the new pam_cracklib module, or that you patched the module if your Linux distribution doesn't include the patched version yet.

The following example shows how to enforce the following password rules:- Minimum length of password must be 8- Minimum number of lower case letters must be 1- Minimum number of upper case letters must be 1- Minimum number of digits must be 1- Minimum number of other characters must be 1

pam_cracklib.so

minlen=8

Minimum length of password is 8

pam_cracklib.so

lcredit=-1

Minimum number of lower case letters is 1

pam_cracklib.so

ucredit=-1

Minimum number of upper case letters is 1

pam_cracklib.so

dcredit=-1

Minimum number of digits is 1

pam_cracklib.so

ocredit=-1

Minimum number of other characters is 1

To setup these password restrictions, edit the /etc/pam.d/system-auth file and add/change the following pam_cracklib arguments highlighted in blue:

Now verify that the new password restrictions work for new passwords. Simply login to a non-root account and change the password using the passwd command. Note that the above requirements are not enforced if you run thepasswd command under root.

NOTE: The /etc/pam.d/system-auth PAM configuration file is auto-generated and contains records which dictate a generic authentication scheme. Keep in mind that authconfig might clobber some changes you made. Since I never run authconfig I usually make changes to this file because it's used by many PAM aware applications. Otherwise I'd have to make changes to many configuration files. Changing system-auth is usually the preferred method. You might even want to disable all execution bits from the /usr/sbin/authconfig binary to prevent authconfig from clobbering your changes.

The pam_unix module parameter remember can be used to configure the number of previous passwords that cannot be reused. And the pam_cracklib module parameter difok can be used to specify the number of characters hat must be different between the old and the new password.

In the following example I will show how to tell the system that a password cannot be reused for at least 6 months and that at least 3 characters must be different between the old and new password.

Remember that in the chapter Enabling Password Aging we set PASS_MIN_DAYS to 7, which specifies the minimum number of days allowed between password changes. Hence, if we tell pam_unix to remember 26 passwords, then the previously used passwords cannot be reused for at least 6 months (26*7 days).

Here is an example. Edit the /etc/pam.d/system-auth file and add/change the following pam_cracklib and pam_unix arguments:

Locking User Accounts After Too Many Login Failures

I do not recommend that the system automatically locks system and shared accounts after too many failed login or su attempts. This could lead to outages if the application's account gets locked due to too many login failures like in this example:

# su oracle -c id
su: incorrect password
#

This could be an easy target for a denial of service attack. At Restricting Direct Login Access for System and Shared Accounts I will show how to disable direct logins for system or shared accounts.

In the following example I will show how to lock only individual user accounts after too many failed su or login attempts.

Add the following two lines highlighted in blue to the /etc/pam.d/system-auth file as shown below:

The first added line counts failed login and failed su attempts for each user. The default location for attempted accesses is recorded in /var/log/faillog.

The second added line specifies to lock accounts automatically after 5 failed login or su attempts (deny=5). The counter will be reset to 0 (reset) on successful entry if deny=n was not exceeded. But you don't want system or shared accounts to be locked after too many login failures (denial of service attack). To exempt system and shared accounts from the deny=n parameter, I added the per_user parameter to the module. The per_user parameter instructs the module NOT to use the deny=n limit for accounts where the maximum number of login failures is set explicitly. For example:

The faillog command with the option "-m -1" has the effect of not placing a limit on the number of failed logins. To instruct the module to activate the deny=n limit for this account again, run:

# faillog -u oracle -m 0

By default, the maximum number of login failures for each account is set to 0 which instructs pam_tally to use the deny=n parameter. The faillog manual page on my Red Hat system says that selecting maximum number of login failures of 0 will deactivate deny=n for the account. The PAM documentation, however, says that per_user will only work if the .fail_max field contains a non-zero value. After testing both values setting it to -1 worked. Maybe because it's read as a high unsigned value?

To see failed login attempts, run:

# faillog

To unlock an account after too many login failures, run:

# faillog -u -r

Make sure to test these changes thoroughly on your system using e.g. ssh and su, and make sure root does not get locked!

To lock/unlock accounts manually, you can run one of the following commands:

# passwd -l
# usermod -L
# passwd -u
# usermod -U

NOTE:

Since the /var/log/faillog is owned by root and only root can write to the /var/log/faillog file, xscreensaver and vlock won't work correctly. Each time xscreensaver or vlock is executed as a non-root user, you won't be able to do an unlock since these programs can't write to /var/log/faillog. I don't have a good solution for that. I can only think of setting the SUID bits on these programs.

Restricting Direct Login Access for System and Shared Accounts

On an audited production system it is very important to know who switched to which system or shared account. Therefore it is prudent to restrict direct logins for all system and shared account where more than one individual knows the password. All users should do a direct login using their own account and then switch to the system or shared account. (If you are just interested in restricting direct root SSH logins, see Securing SSH.)

However, there are situations where you have to allow direct logins for system or shared accounts. For example, within an Oracle RAC cluster you have to enable direct ssh logins for oracle. But in such an environment you have to protect the whole cluster as a single entity against incoming ssh connection, i.e. direct oracle logins should not work if you come from a node that is not part of the cluster. In the following example I will show how to achieve this goal as well.

Usually all system and shared accounts have one thing in common, that is they are not in the "users" group. The following example assumes that all individual user accounts are in the "users" group but system and shared accounts like root and oracle are not. If you want to go a step further, a good solution would be to implement a new 'logingroup' users group which would require users to be given explicit access.

To accomplish this goal I will add the pam_access module to the PAM configuration files listed above. This module provides logdaemon-style login access control based on login names, host names, IP addresses, etc. The PAM module type that has to be used in the configuration files is account. This module type does the authorization, i.e. is the user allowed to login (e.g. time, day)? Don't confuse the PAM module type account with auth which does the authentication, for example checking the password. And the control flag I will use is required. It specifies that Success is required, Failure means that it will still call the remaining modules, but the result is already determined.

For SSH Logins add the pam_access module to /etc/pam.d/sshd as follows:

Now add the following line to the /etc/security/access.conf configuration file:

-:ALL EXCEPT users :ALL

The /etc/security/access.conf configuration file is read by the pam_access module. This entry specifies that no users are accepted except users that are in the "users" group. Since the pam_access module has been configured for "Authorization" (account) in the above PAM configuration files, it denies direct logins for all accounts except the ones that are in the "users" group.

Now on some systems like Oracle RAC clusters you have to enable direct ssh logins for oracle within the cluster. On such systems you can enable direct ssh logins for oracle within the cluster by adding/changing the following lines in /etc/security/access.conf:

The first line has been edited to include the oracle account which will allow general direct logins. However, the second line specifies that direct logins for oracle are only allowed from Oracle RAC nodes (rac1cluster, rac2cluster, and rac3cluster) that are part of the cluster.

This means that the above entries in /etc/security/access.conf will stop cron from working. Note that it is very prudent to always check whether pam_access is configured for any other service on the system!

To ensure that all users on the system can still run cron jobs you can add the following argument to pam_access in /etc/pam.d/crond:

This ensures that the /etc/security/access.conf configuration file is not invoked by crond. Since pam_cracklib does not grant permissions if the configuration file does not exist, execute the following command to create an empty file:

# touch /etc/security/access-cron.conf

Now verify that cron jobs can be launched by any user on the system.

NOTE:

The above example will only work if there exists no "users" account in the /etc/passwd file on the system, which is usually the case. Otherwise you have to either delete the "users" account or you have to designate or create another group name.

Restricting su Access to System and Shared Accounts

This chapter shows how to restrict people from su-ing to system and shared accounts even if they know the passwords.

Users usually don't share the passwords of their own accounts but are less hesitant to share it for shared accounts. This chapter helps to mitigate this problem.

The following example shows how to restrict su access to the root, oracle, and postgres account to a specific set of users.

NOTE: The documentation about the pam_wheel module included in many Linux distributions is wrong. For instance, in Red Hat Advanced Server 2.1 the pam_wheel module does not only restrict people from su-ing to the rootaccount like it used to be. It restricts people from su-ing to any account.

Add all users who are allowed to su to the root, oracle, and postgres account to the new member groups created above.The following requirement will be configured:- Only admin1 should be able to su to root, oracle, and postgres.- Only oracledba1 should be able to su to oracle.- Only postgresdba1 should be able to su to postgres.- No one else on the system should be able to su to any account.

As you probably noted, I did not add adminuser1 to the other member groups. Instead, I will show how to give people in the rootmembers group automatically su access to the oracle and postgres account without adding them to the oraclemembers and postgresmembers groups. I consider root admins an exception. They should not be added to all member groups on the system.

Next add the three authentication lines highlighted in blue to the /etc/pam.d/su file as shown below:

These additional authentication lines specify that nobody should be able to su to any account unless at least one of the PAM services, su-root-members or su-other-members, returns Success. The control flag sufficient means that a Success will bypass the remaining authentication modules and overall Success is returned for the authentication part. Failure means that the failed authentication PAM service is ignored. If both authentication PAM services fail, then the last authentication module pam_deny is invoked which will deny all requests for any available authentication module. This will cause the authentication part to fail for the su command.

Next the new authentication PAM service configuration files /etc/pam.d/su-root-members and /etc/pam.d/su-other-members need to be created.

The file /etc/pam.d/su-root-members referenced in /etc/pam.d/su should read like:

The file /etc/security/su-rootmembers-access referenced in /etc/pam.d/su-root-members should read like:

root
oracle
postgres

The control flag required which is specified for both modules means that both modules have to return Success. Otherwise this PAM service will return Failure to the "su" PAM service configured in /etc/pam.d/su. The first line returns Success only if the user is in the rootmembers groups. The second line allows only access (sense=allow) to those users specified in /etc/security/rootusername, which is root, oracle, and postgres - these are the only users that will be accepted as a user argument to su. The item=user argument instructs pam_listfile that the entries in /etc/security/rootusername are usernames. If an error occurs, such as an unreadable configuration file, access is denied (onerr=fail).

NOTE: Once su access to root is working for users in the rootmembers, I recommend to avoid making any changes to the /etc/pam.d/su-root-members file in the future. Making a mistake in this file could revoke access to root for all users on the system. That's the reason why I created two PAM service files, /etc/pam.d/su-root-members for people in the rootmembers group, and /etc/pam.d/su-other-members (see below) for all other member groups since you will most probably add more member groups to this file in the future.

Next the file /etc/pam.d/su-other-members referenced in /etc/pam.d/su should be created and read like:

If one of the two PAM services returns Success, it will return Success to the "su" PAM service configured in /etc/pam.d/su. Otherwise the last module will be invoked which will deny all further requests and the authentication fails.

Next the PAM services "su-oracle-members" and "su-postgres-members" have to be created.

The file /etc/pam.d/su-oracle-members referenced in /etc/pam.d/su-other-members should read like:

The file /etc/security/su-postgresmembers-access referenced in /etc/pam.d/su-postgres-members should read like:

postgres

Now verify that adminuser1 can su to root, oracle, and postgres. No one else should be able to su to root. oracleuser1 should be able to su to oracle only, and postgresuser1 should be able to su to postgres only. No one else on the system should be able su to any of these accounts even if they know the password.

Preventing Accidental Denial of Service

Linux allows you to set limits on the amount of system resources that users and groups can use. This is also very handy if bugs in programs accidentally use up too much resources, slow down the machine, or even render the system unusable. I've seen systems where incorrect settings have allowed programs to use up too much resources which made the server unresponsible for new connections or local logins (e.g. a program uses up all file handles on the system). This could become a security issue if someone is allowed to use up all resources and causes a denial of service attack. Depending on your environment you may want to review resource limits for user accounts and groups.

The following example shows a practical use of setting or restricting system resources for an Oracle user account. For a list of system resource settings, see /etc/security/limits.conf. It would be a good idea to review the default settings of system resource.

Most shells like Bash provide control over various resources like the maximum allowable number of open file descriptors or the maximum number of processes available to a user. To see all shell limits, run:

ulimit -a

For more information on ulimit for the Bash shell, see man bash and search for ulimit.

Important Note:Setting "hard" and "soft" limits in the following examples might not work properly when you login to oracle using a SSH session. It should work if you log in as root and su to oracle. Resource limits should also work if the application is started automatically during the boot process. But if you experience the problem that the changed resource limits in /etc/security/limits.conf are not applied when logging in through SSH, then you may have to try to set UsePrivilegeSeparation in /etc/ssh/sshd_config to "no" and restart the SSH daemon by executing /etc/init.d/sshd restart. Unfortunately, privilege separation does not work properly with PAM on some Linux distributions. But also note that turning off privilege separation is not really recommended since it's a valuable security feature that has already prevented exploitation of SSH vulnerabilities.

For example, to change the number of file handles or open files that the Oracle user can use, you have to edit the file /etc/security/limits.conf as root and make the following changes or add the following lines, respectively:

oracle soft nofile 4096
oracle hard nofile 63536

The "soft limit" in the first line defines the number of file handles or open files that the Oracle user will have after login. If the Oracle user gets error messages about running out of file handles, then the Oracle user can increase the number of file handles like in this example up to 63536 ("hard limit") by running the following command:

ulimit -n 63536

You can set the "soft" and "hard" limits higher if necessary. Note that I do not recommend to set the "hard" limit for nofile for the oracle user equal to /proc/sys/fs/file-max. If you do that and the oracle user uses up all the file handles, then the whole system will be out of file handles. This could mean that you won't be able to initiate new remote logins any more since the system won't be able to open any PAM modules which are required for performing a login.

You also need to ensure that pam_limits is configured in the file /etc/pam.d/system-auth, or in /etc/pam.d/sshd (for SSH), /etc/pam.d/su (for su), or /etc/pam.d/login (local logins and telnet) if you don't want to enable it for all logins, or if /etc/pam.d/system-auth does not exist like on SUSE. This is the PAM module that will read the /etc/security/limits.conf file. The entry should read like:

session required /lib/security/pam_limits.so

Here are the two "session" entries I have in my /etc/pam.d/system-auth file:

Now login to the oracle account again since the changes will become effective for new login sessions only.

$ su - oracle
$ ulimit -n
4096
$

Note that the ulimit options are different for other shells.

The default limit for oracle is now 4096 and the oracle user can increase the number of file handles up to 63536:

$ su - oracle
$ ulimit -n
4096
$ ulimit -n 63536
$ ulimit -n
63536
$

To make this change permanent, add "ulimit -n 63536" (for Bash) to the ~oracle/.bash_profile file which is the user startup file for the Bash shell on Red Hat Linux (to verify your shell run: echo $SHELL). To do this you could simply copy/paste the following commands for the oracle's Bash shell:

su - oracle
cat >> ~oracle/.bash_profile << EOF
ulimit -n 63536
EOF

Displaying Login Banners

It is prudent to place a legal banner on login screens on all servers for legal reasons and to potentially deter intruders among other things. Consult legal counsel for the content of the banner.

If you want to print a legal banner after a user logs in using ssh, local console etc., you can use the /etc/motd file. Create the file if it doesn't exist and type in the banner that applies to your organization.

# cat /etc/motd
This system is classified...
Use of this system constitutes consent to official monitoring.
#

For SSH you can edit the Banner parameter in the /etc/ssh/sshd_config file which will display the banner before the login prompt.

For local console logins you can edit the /etc/issue which will display the banner before the login prompt.

For GDM you could make the following changes to require a user to acknowledge the legal banner by selecting 'Yes' or 'No'. Edit the /etc/X11/gdm/PreSession/Default file and add the following lines at the beginning of the script:

You have to add a sleep of 10 seconds, otherwise GDM will believe that X crashed if the session lasted less than 10 seconds. Unfortunately, at the time of this writing the 10 seconds timeout is hardcoded and there is no configuration parameter to change it - I checked the source code.

Miscellaneous

Before you put a server into production or better, before you put a server on the network, you should have an integrity checker installed on your system that lets you check if unauthorized changes have been made. In this way, if an intruder compromises your system you will know what changed on your server. You should also have an Intrusion Detection Software (IDS) solution in place that alarms you about intrusions, as well as Intrusion Prevention software.

It is outside the scope of this article to cover Linux Monitoring and Intrusion Detection solutions. There are lots of interesting articles out there to read and there are several good products available on the market including open source solutions. For a powerful open source network-intrusion prevention and detection system, see Securing your system with Snort.

who Shows a listing of currently logged-in users. w Shows who is logged on and what they are doing. last Shows a list of last logged-in users, including login time, logout time, login IP address, etc. lastb Same as last, except that by default it shows a log of the file /var/log/btmp, which contains all the bad login attempts. lastlog This command reports data maintained in /var/log/lastlog, which is a record of the last time a user logged in. ac Prints out the connect time in hours on a per-user basis or daily basis etc. This command reads /var/log/wtmp. dump-utmp Converts raw data from /var/run/utmp or /var/log/wtmp into ASCII-parsable format.

Also check the /var/log/messages file.

Other

The following items may not necessarily be Linux security related but should be configured correctly on all audited Linux systems:

Redmi 4

About Me

Hi This is Suresh Kumar Pakalapati . I am a person who is positive about every aspect of life. There are many things I like to do, to see, and to experience. I like to feel the music flowing on my face, I like good books and romantic, action,cartoon movies. I like the land and the nature, And, I like to keep smile on my face. I always wanted to be a great and successful person in the world and I know it is needed More and More education, More work, Success Never come in a short Time.....